Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick. The cylindrical shell is closed at its top end by a removable top head. The top head is removable so that components, such a fuel bundles, located in the RPV can be accessed. The RPV cylindrical shell is closed at its bottom end by a dome shaped bottom head assembly welded to the shell.
A plurality of openings, sometimes referred to as penetrations, are formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. Typically, a substantially cylindrical stub tube having a bore extending therethrough is welded to the bottom head dome and the tube bore aligns with an opening in the bottom head dome. The cylindrical stub tube typically is fabricated from a corrosion resistant material such as stainless steel or Ni--Cr--Fe.
With a control rod drive assembly, for example, the control rod drive housing, e.g., a tube, is inserted through the bottom head dome opening and stub tube bore, and the housing extends into the RPV. The control rod drive (CRD) housing is welded to the stub tube to maintain the housing in the desired position. The stub tube thus serves as a transition piece between the bottom head dome, which typically is fabricated from low alloy steel (LAS), and the CRD housing, which typically is fabricated from stainless steel.
Stress corrosion cracking (SCC) is a known phenomenon occurring to adjacent stub tube welds connecting the bottom head dome to the stub tube and connecting the stub tube to the CRD housing. Particularly, the stub tube welds are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. Such stresses may, at times, cause cracks adjacent the stub tube welds or may even cause cracks in the weld materials.
To avoid SCC, it is desirable to eliminate welds. In addition, the welds which are required should be inspectable. It would be desirable to provide a stainless steel penetration to LAS head attachment that eliminates the stub tube to head weld, meets all design stress requirements, and simplifies fabrication.